A liquid crystal display device employs liquid crystals to display images. Typically it is composed of a liquid crystal panel, a backlight module, related digital circuits and a power supply. Backlight module can be divided into three types in terms of a position of a light tube: 1) Side lighting type structure which a light source is disposed by a lateral side of a light guide plate; 2) Direct lighting type structure which light is emitted from a spontaneous light source and is reflected by a reflecting plate to travel upward, and is emitted from a front after it is diffused evenly by a diffusing plate; 3) Hollow lighting type structure which a hot-cathode tube is used as a light source, this structure uses air as a media for light. A light pointed downward is upwardly reflected by a prism and a reflecting plate. A part of light travels upward to go through a light guide plate and is emitted from the surface of the light source, and another part of light enters into a cavity again because of total reflection, then is emitted through the light guide plate after being refracted and reflected; however, a light pointed upward enters into the light guide plate and is emitted directly, or is emitted after refractions and reflections, and the light guide plate is a wedge structure.
A conventional side lighting type backlight module employs LED as a light source. Referring to FIGS. 1a and 1b, a LED light bar 1 is fixed on a horizontal wall of a heat dissipating piece 2 by screws going through fixing holes disposed on the horizontal wall. The heat dissipating piece 2 is then fixed on a back plate 3 by screws going through fixing holes disposed on a vertical wall of the heat dissipating piece 2. This installation method for the LED light bar 1 requires a number of procedures and its labor cost is high; in addition, it also has the following drawbacks:
If heat conductive adhesive is used for fixing the LED light bar 1, it will be difficult to dismount the LED light bar 1 if it needs to be reworked, even if it can be dismounted, the LED light bar 1 will be deformed easily and will cause defects in later utilization.
If the LED light bar 1 is fixed by screws, LED pitches will be inconsistent because some places have screws while some do not; therefore, a dot pattern of a light guide plate (not shown) has to be reinforced for locations with the screws, in order to prevent dimmed blocks shown on a display area of the module corresponding to the locations of the screws because of the inconsistent LED pitches. It is a drawback for positioning of the light guide plate and its dot pattern design. Furthermore, because the LED pitches of the locations with and without the screws are inconsistent, thus a light mixing distance in the module design has to be calculated based on large LED pitches, this causes an actual light mixing distance bigger than normal. In addition, heads of the screws are exposed outside between the LEDs, therefore intervals between two soldering leads of LED electrodes and the exposed head are very small, which can cause safety problems of short circuit or high voltage sparking easily. Therefore, some provide a solution by fixing the LED light bar on its reverse side by screws, that means, blind holes with threads on a PCB of the LED light bar are locked by screws reversely, but a thickness of the PCB of the LED light bar has to be increased or else it will be impossible for the screws to be locked tightly, it will increase the costs if the PCB of the LED light bar is made thicker.